1. Field of the Invention
The present invention relates to object sensing apparatuses and, more particularly, to an object sensing apparatus for accurately sensing an object, such as a human body or the like, approaching an automatic door or the like.
2. Background of the Invention
Conventionally, there is known a trigonometric distance measuring device in which light projected from a light projecting element, such as an infrared-ray diode, is projected through a projecting lens, and light reflected from an object to be sensed, such as a human body, is received by a light receiving element, such as a PSD (position sensitive detector) placed behind a light receiving lens at a predetermined distance from the light projecting element.
An object sensing apparatus for a safety apparatus utilizing such conventional distance measuring device is also known. The distance measuring device is disposed, for example, in the vicinity of an automatic door, or is attached to the door itself. By this construction, when the object sensing apparatus senses an object, such as a human body, passing or approaching the automatic door, the distance measuring device measures the distance from the door to the object and an appropriate determination is made to open or close the door, or to prevent the door from closing when someone is passing through.
FIG. 7 shows a conventional object sensing apparatus employing a distance measuring device a of the type described above. Using this conventional apparatus, it is impossible to obtain an accurate distance measurement of an object T at a particular distance Z. The distance measuring device a is attached to an installation site S in a retracted location thereof at a distance c from a front end surface b of the installation site, thereby providing a cavity d in front of the distance measuring device a. With this structure, there is a necessity that a distance measurement be positively made on a left side of the front end surface b or over an entire range where there is a possibility that an object or an individual may approach. If the distance measuring device a is installed at a retracted location as set forth above, dust or the like tends to accumulate within the cavity d or in the vicinity of an opening of the installation site S. In order to avoid this, and thereby prevent malfunction of the object sensing apparatus, a protection cover, filter or the like (hereinafter referred to as a filter member e) is provided at a location close to the front end surface b of the installation site S.
A particular example where a filter member is provided in front of a distance measuring device as described above is in an auto-focusing camera. If the filter member e is instead provided at a distance of approximately c from the front end surface b of the installation site S and in parallel relation with the distance measuring device a, a light beam La projected from a light projecting element f of the distance measuring device a passes through a light projecting lens g and is projected onto the object T to be sensed. A light component Lb reflected by the object T then transmits through a light receiving lens h and is received by a light receiving element i of the distance measuring device a. In this process, part of the light beam La is scattered by an inner surface of the filter member e, causing a light component Lc. Also, a light beam Ld transmitted through an end of the light projecting lens g similarly causes a reflected light component Le or a scattering light component Lf by the inner surface of the filter member e. These light components transmit through the light receiving lens h and are received by the light receiving element i. This extra reflected or scattering light, when received by the light receiving element i, causes error in a measurement result obtained by the distance measuring device a, thereby rendering accurate distance measurements impossible.
A detailed explanation of light reflected by an inner surface of the filter member e is provided with reference to FIG. 8(a). The light projecting element f is usually arranged behind a focal point F of the light projecting lens g so that an image thereof is focused at a point distant by a predetermined distance from the light projecting lens g. The light transmitting through the light projecting lens g involves light beams from secondary light sources, such as reflection by a substrate fixed with the light projecting element f, a stem, and a surface of a case, in addition to direct light from the light projecting element f. If an outer peripheral point thereof is taken at P, the point P is focused at Pa due to light beams Ld,Lg transmitting through the light projecting lens g.
With the filter member e arranged in parallel with a plane vertical to the light projecting axis of the light projecting element f, a study was conducted on the reflecting direction of light from light components Ld,Lg which is reflected by the inner surface of the filter member. The study was conducted for respective cases where a filter member el was at a location closer to the light projecting lens g than a location of the filter member e, and where a filter member e2 was at a location more distant from the light projecting lens g than a location of the filter member e. Scattering light from the reflecting surface was considered negligible and excluded from the study.
The light components resulting from reflection of the light component Ld and the light component Lg by the inner surface of the filter members e1, e, e2 are Le1,Lh1, Le,Lh and Le2,Lh2, respectively.
The reflected light components Le1 and Lh1 by the filter member e1 closest to the lens did not transmit through the light receiving lens h and did not arrive at the light receiving element i, thereby having no effect upon a measurement result.
However, the reflected light components Le and Lh by the filter member e both transmitted through the light receiving lens h and arrived at the light receiving element i, thereby having an effect upon a measurement result.
The reflected light Le2 by the filter member e2 did not transmit through the light receiving lens h and did not arrive at the light receiving element i. However, the reflected light Lh2 transmitted through the light receiving lens h and arrived at the light receiving element i, thereby having an effect upon a measurement result.
The degree of effect on a measurement result depends upon various conditions such as an aperture of the light projecting/receiving lens, the base line length (distance between the light projecting and receiving lenses), the size of the light projecting element, the location of the light projecting element (distance from the lens), the magnitude of secondary light, the condition of the reflecting surface of the filter member (magnitude and direction of diffusing reflection due to difference in smoothness), the size and arrangement of the light receiving element, and the location of the reflecting surface of the filter member. However, qualitatively, as shown in FIG. 8(b), where the magnitude of an effect on a measurement result is taken in a vertical axis and a distance from the lens is taken in a horizontal axis, there is almost no effect on a measurement result for the filter member e1 located at a distance R from the lens. In contrast, the effect on measurement result gradually changes as the distance R is exceeded and the lens becomes distant. The effect becomes maximum at the location of the filter member e, it decreases as the distance from the lens increases, and it becomes small at a location of the filter member e2.
In view of the foregoing, there is a necessity to place the entire range that a passer or object may approach at a distance between the filter member and the lens in order to positively perform distance measurement. If the filter member e is placed in parallel relation with a plane vertical to the light projecting axis, errors occur in the measurement result, thereby rendering it difficult to obtain an accurate distance measurement.